Electropolishing apparatus

ABSTRACT

An electropolishing apparatus comprises a direct current electrical power supply and reservoirs of electrolyte and coolant, a lead which carries current to a cathode block and pumps which pump the electrolyte and coolant from their respective reservoirs through tubes to the block which is made of brass and is covered with an electrolyte absorbent boot knitted from Teflon coated fiber glass thread.

FIELD OF THE INVENTION

This invention relates to the cleaning and polishing of metal surfaces by electrochemical action.

BACKGROUND OF THE INVENTION

Electropolishing is the controlled electrochemical removal of surface metal resulting in a brilliant appearance and improved properties of the metal. The process is sometimes called “reverse plating” or “super passivation” and has a leveling effect which produces a smooth surface with increased reflectivity. With stainless steel welds the deformed amorphous outer layer of the metal is removed leaving a chromium rich passive surface free of embedded contaminants and work induced residual stresses.

Electropolishing draws surface contaminants away from the workpiece as well as removing micro projections resulting in micro leveling of the surface greatly improving resistance to bacteria and corrosion. Most metals can be successfully electropolished but the best results are obtained with metals that have fine grained boundaries which are free of non metallic inclusions and seams.

Metals which have a high content of silicon, lead or sulphur are usually troublesome. Electropolishing is most commonly used on stainless steel but aluminium, brass and carbon steels may also be electropolished as well as copper, beryllium copper, bronze, nickel, nickel silver, wrought aluminium alloys, titanium and gold.

The final quality and consistency of the finish is determined by controlling the voltage and current density of the DC power supplied and thereby the temperature of the surface of the workpiece. The appropriate electrolyte for the particular metal must be used and is normally a liquid solution but a gel form may also be used.

Conventional electropolishing has been achieved by immersing the work piece connected to the positive terminal of a DC power supply thereby becoming an anode, in an electrolyte bath. Metal plates in the bath are connected to the negative terminal of the DC power supply thereby becoming the cathode for ionic conduction which removes unwanted particles from the anodic work piece. Fine burrs become high current density areas and are removed while smooth sections become low current density areas and are polished. The result is an overall reduction of the micro surface profile with a simultaneous smoothing and brightening of the metal surface.

In the case of stainless steel alloys, iron and nickel atoms are more readily extracted from the crystal lattice than are chromium atoms. Accordingly the process removes iron and nickel preferentially leaving a surface rich in chromium and chromium oxides resulting in passivation of the surface. The oxide rich layer reduces the absorption of moisture which prevents corrosion and maintains a clean surface.

However this conventional method requires baths which are larger than the workpiece and multiple baths for the different electrolytes required for each metal to be polished since the replacement of large volumes of electrolyte in a single bath is costly. Further the baths consume considerable power and produce large amounts of toxic waste.

Accordingly a number of attempts have been made to develop a method of electropolishing which does not require the immersion of a workpiece in a permanently located electrolyte bath but which can be done by portable apparatus. For example U.S. Pat. No. 6,203,689 teaches a mobile apparatus for polishing an inner face of a deep hole of an article comprising a support for holding an article so that the article is maintained erect in an electrolytic bath while an electrode is inserted in the deep hole of the article. However this apparatus is complicated and clearly only suitable for polishing the inner face of articles with a deep hole.

Similarly U.S. Pat. No. 5,507,923 teaches an apparatus for polishing the inside surface of tubular products such as rifled stock, or firearm or artilliary barrels and so is limited to tubular products. U.S. Pat. No. 2,539,455 teaches the use of a cathode wand covered with a layer of electrically non conducting liquid retentive fabric material wetted with an electrolyte. However the fabric needs to be continually wetted with the electrolyte manually and the anode current is limited by the temperature which the fabric can withstand. This is relatively low and so the polishing process can be very slow.

U.S. Pat. No. 4,431,501 attempts to overcome the cathode heating problem of U.S. Pat. No. 2,539,455 by providing water pipes which cool the current coil contained in a cathode box. However the specification does not clearly teach how the cooling water pipes achieve cooling of the cathode and it is doubtful that high current ratings could be sustained to ensure rapid polishing. Further it appears that the electrolyte is gravity fed to the absorbent soft surface of the cathode box which limits the working orientation of the latter ie the box must always be above the work piece and the rate of supply of electrolyte is limited by the gravity feed.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide apparatus which overcomes the above problems of the prior art or at least provides an alternative method of electropolishing.

STATEMENT OF THE INVENTION

According to the present invention an electropolishing apparatus comprises a direct current electrical power supply and reservoirs of electrolyte and coolant a lead which carries current to the underside of a cathode block and pumps which pump the electrolyte and coolant from their respective reservoirs through tubes to the cathode block.

Preferably the cathode block is made of brass.

Preferably the cathode block is covered with an electrolyte absorbent boot.

Preferably the boot is knitted from Teflon coated fibre glass thread.

Preferably the electrolyte is a solution comprising about 45% of orthophosphoric acid and about 25% citric acid.

Preferably the electrolyte also comprises up to 2% of a bonding agent.

Preferably the coolant is high density glycol.

Preferably the power supply is operated in a range from 20 to 60 volts direct current.

Preferably the power supply has a slightly alternating current component on top of the direct current.

Preferably the cathode block is attached to a handle by means of three copper tubes one of which feeds electrolyte to the block and the other two of which provide a circuit for the coolant to flow through the block.

Preferably the handle has an on/off control for the supply of electrolyte to the cathode block.

Preferably the current lead connects to all three copper tubes.

Preferably the apparatus has a switch to turn the power supply on.

Preferably the apparatus has a switch to switch the electrolyte supply from automatic to manual.

Preferably the apparatus has a controller to control the rate of flow of electrolyte to the cathode block,

Preferably the apparatus has an immersion water heater to recondition the boot before use.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is now described by way of example only with reference to the accompanying drawings in which

FIG. 1 is a perspective view from above of an electropolishing apparatus

FIG. 2 is a rear perspective view of the apparatus in FIG. 1

FIG. 3 is an underside view of the anode block

FIG. 4 is a diagram showing the coolant and electrolyte streams into and out of the cathode block and the electrical current supply.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 portable unit 1 houses a power supply (not shown) and polishing cathode block 2 attaches to handle 3. Handle 3 is connected to unit 1 by lead 4 which houses coolant circuit tubes, electrolyte supply tube and electrical current cable (not shown). Bottle 5 supplies the electrolyte and bottle 6 is a reservoir for the coolant.

Box 7 houses pinch tube pump 8 which pumps electrolyte from bottle 5 to cathode block 2, switch 9 switches pump 8 from automatic to demand and knob 10 regulates the rate of electrolyte flow. Button 11 on handle 3 operates the flow of electrolyte when pump 8 is switched to demand. Switch 12 turns the power supply on and grate 13 is an exhaust for cooling fans inside unit 1. Cup 14 is an immersion heater which accommodates cathode block 2. FIG. 2 shows box 15 housing pinch tube pump 16 which pumps coolant from bottle 6 through lead 4 to cathode block 2 and back through lead 4 to bottle 6.

FIG. 3 shows the detail of cathode block 2. Central copper tube 20 receives electrolyte from bottle 5 which then drips out of holes 21 in the underside of block 2. Outer copper tube 22 receives coolant from bottle 6 which circulates around the perimeter of block 2 and leaves via outer copper tube 23 to return to bottle 6. All three copper tubes are connected to the electrical cable (not shown) in lead 4. Boot 24 is knitted from teflon coated fibre glass thread and is retained on block 2 by draw thread 25. Boot 24 is therefore kept wetted by electrolyte dripping from holes 21.

Schematic FIG. 4 illustrates flows to and from block 2. Flow arrows 30 indicate the flow of electrolyte from bottle 5 via pump 7 to block 2 while flow 31 indicates the coolant circuit from bottle 6 via pump 15 through a perimeter channel (not shown) in block 2 and back to bottle 6. Electrical current is supplied from the negative terminal of power supply 32 in unit 1 to all three copper tubes 21, 22 and 23 in handle 11. The work piece (not shown) is connected to the positive terminal of power supply 32.

Accordingly the electropolishing process is achieved as follows. The work piece is connected to the positive terminal of power supply 32 in unit 1 which is then turned on by switch 12. Electrolyte flow 30 is switched to demand by switch 9 and the flow rate is set by turning knob 10. Block 2 is then held on the area of the work piece to be polished and button 11 on handle 3 is operated to apply electrolyte to boot 24.

In addition to the improved construction of the unit there are also a number of other improvements. The electrolyte is a solution of about 45% ortho-phosphoric acid and about 25% citric acid which has been found to produce a better result on stainless steel than the conventional phosphoric acid solution. Also a small alternating current component has been applied on top of the DC power supply since this small alternating pulsation of current has been found to be beneficial.

Most importantly block 2 can be operated at about 2400 watts which is more effective for polishing because of the efficient cooling of block 2. The coolant is a high density glycol which is cooled by fans inside unit 1. Boot 24 is knitted from Teflon coated fibre glass which is longer lasting than other electrolyte absorbent materials as it can withstand higher temperatures. The boot becomes stiff and non absorbent if excessive heat boils off the water in the electrolyte solution but can be reconditioned by immersion in boiling water in cup 14.

Variations

It will be realised that the foregoing has been given by way of illustrative example only and that all other modifications and variations as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth. Block 2 can be formed in different shapes suitable for different shaped work pieces eg a semi tubular shape for internal tubular work pieces.

Throughout the description and claims of this specification the word “comprise” and variations of that word such as “comprises” and “comprising” are not intended to exclude other additives, components, integers or steps. 

1. An electropolishing apparatus comprising a direct current electrical power supply and reservoirs of electrolyte and coolant, a lead which carries current to the underside of an a cathode block and pumps which pump the electrolyte and coolant from their respective reservoirs through tubes to the block.
 2. The apparatus of claim 1 in which the block is made of brass.
 3. The apparatus of claim 1 in which the block is covered with an electrolyte absorbent boot.
 4. The apparatus of claim 3 in which the boot is knitted from Teflon coated fibre glass thread.
 5. The apparatus of claim 1 in which the electrolyte is a solution comprising about 45% of orthophosphoric acid and about 25% citric acid.
 6. The apparatus of claim 5 in which the electrolyte also comprises up to 2% of a bonding agent.
 7. The apparatus of claim 1 in which the coolant is high density glycol.
 8. The apparatus of claim 1 in which the power supply is operated in a range from 20 to 60 volts direct current.
 9. The apparatus of claim 1 in which the power supply has a slightly alternating current component on top of the direct current.
 10. The apparatus of claim 1 in which the block is attached to a handle by means of three copper tubes one of which feeds electrolyte to the block and the other two of which provide a circuit for the coolant to flow through the block.
 11. The apparatus of claim 10 in which the handle has an on/off control for the supply of electrolyte to the block.
 12. The apparatus of claim 10 in which the current lead connects to all three copper tubes.
 13. The apparatus of claim 1 which has a switch to turn the power supply on.
 14. The apparatus of claim 1 which has a switch to switch the electrolyte supply from automatic to manual.
 15. The apparatus of claim 1 which has a controller to control the rate of flow of electrolyte to the block,
 16. The apparatus of claim 3 which has an immersion water heater to recondition the boot before use. 